Part of PAZ-996.

Part of PAZ-996.

Add support for CC By-NC-SA. This is not actually used by the East
London dataset, but it's a perfectly cromulent Creative Commons
licence. (My guess is that the uses CC By-ND-SA are typos for this,
since that licence is invalid but used, this one is valid but unused,
and they are only two bits, one code-point or one key-position
different.)